Magnetic resonance imaging (MRI) is a medical imaging technique in widespread use for viewing the structure and/or function of the human body. MRI systems provide soft-tissue contrast, such as for diagnosing many soft-tissue disorders. MRI systems generally implement a two-phase method. The first phase is the excitation phase, in which a magnetic resonance signal is created in the subject with a main, polarizing magnetic field, B0, and a radio frequency (RF) excitation pulse, B1+. The second phase is the acquisition phase, in which the system receives an electromagnetic signal emitted as the excited nuclei relax back into alignment with the main magnetic field after the excitation pulse B1 is terminated. These two phases are repeated pair-wise to acquire enough data to construct an image.
Higher magnetic field strength scanners are used to improve image signal-to-noise ratio and contrast. However, a spatial variation in the magnitude of the RF excitation magnetic field, B1+, occurs with higher main magnetic field strengths, such as 7 Tesla. This undesirable non-uniformity in the excitation across the region of interest is commonly referred to as “center brightening,” “B1+ inhomogeneity” or “flip angle inhomogeneity.”
Newer-generation MRI systems generate RF pulses with a spatially tailored excitation pattern to mitigate B1+ inhomogeneity by exciting a spatial inverse of the inhomogeneity. In these systems, multiple radio-frequency pulse trains are transmitted in parallel over independent radio-frequency transmit channels, e.g., the individual rods of a whole-body antenna. This method, referred to as “parallel transmission” or “parallel excitation,” exploits variations among the different spatial profiles of a multi-element RF coil array. Parallel excitation enables several important applications beyond the mitigation of B1+ inhomogeneity, including flexibly shaped excitation volumes.
Parallel transmission is also used to reduce acquisition times. The variation in sensitivities between individual coils in the array is exploited to reduce the number of gradient encodings involved in an imaging procedure.
Reduced acquisition times and other advances enable more dynamic MRI procedures directed to studying the motion of an object. For example, dynamic MRI imaging provides for cine imaging of the heart. The reduced acquisition times maintain image quality despite the reduced number of gradient encodings.
The temporal resolution of dynamic imaging can nonetheless benefit from further decreases in acquisition time. A compromise is often made between spatial resolution and temporal resolution. Undersampling techniques are usually implemented to achieve further savings in acquisition time. For example, scan data may be acquired for every other line to cut the acquisition time in half. Undersampling may result in less desirable spatial resolution.